Ductwork Sealing Guide

Duct leaks are most common at joints: where duct sections connect, where branch lines connect to the main trunk, and at register boots. Start by turning the HVAC fan to ON and feeling along duct seams for air escaping. For hard-to-spot leaks, use a smoke pencil (or incense stick) near joints—smoke will move toward escaping air. In the attic or crawlspace, look for disconnected sections, rodent damage, crushed ducts, and gaps around duct boots at floor registers. Use a flashlight inside the duct at a joint and look from the outside for light shining through. Also check the return duct system—leaks here pull unconditioned attic or crawlspace air into the system. The return side leaks are often more damaging than supply side leaks.

Mastic is a thick, water-based adhesive that dries to a rubber-like seal. Purchase a 1-gallon bucket of mastic and a cheap paintbrush. Clean the duct surface around each joint—mastic does not stick to dirty or greasy surfaces. For small gaps (under 1/8 inch), apply mastic directly over the seam with a brush. For larger gaps, press fiberglass mesh tape into the gap first, then apply mastic over the tape. Apply mastic at least 1/8 inch thick over the entire joint, extending 1 inch beyond the seam on each side. Work methodically: start at the furnace, follow the main trunk, then each branch. Seal the plenum connection to the furnace first—this is often the leakiest joint. Allow mastic to dry 24 hours before operating the system.

Flexible ducts are commonly used for branch runs to individual rooms. They are prone to leaks at connection points where the flex duct attaches to the metal trunk line and the register boot. At each connection: slide the flex duct inner plastic liner over the metal fitting, secure with a zip tie or worm-drive clamp. Pull the fiberglass insulation over the connection, then seal the outer plastic vapor barrier with UL-181 rated tape or mastic. Ensure the flex duct is not crushed or kinked—flex duct must be supported every 4 feet and have gentle bends (minimum radius = 1x the duct diameter). Kinked or sagging flex duct restricts airflow and negates the benefits of sealing.

The register boot (the box behind the floor or ceiling vent) is a common leak point. The gap between the boot and the drywall or subfloor allows conditioned air to escape into the wall cavity or unconditioned space. Remove the register cover. Apply mastic to the joint where the boot flange meets the drywall or floor. For added sealing, use foam gasket tape between the register cover and the finished surface. Also seal the joint where the duct connects to the boot—this is often a simple friction fit that leaks copiously. The supply plenum (the box on top of the furnace) and return plenum (below or beside the furnace) are the highest-pressure points in the duct system. Seal every seam in the plenum with mastic. Unsealed plenum connections can leak 20% or more of total airflow.

For ducts in inaccessible areas (inside walls, between floors, or under slabs), professional sealing is the only option. Two methods exist: aerosol-based sealing (Aeroseal) injects a sealant mist into the pressurized duct system—particles accumulate at leak points and seal them from the inside. This method reaches 95%+ of leaks. Traditional hand sealing is used for accessible ducts. Professional duct sealing costs $500–$1,500 depending on home size and leak severity. Professional sealing is often incentivized by utility rebates—check with your local utility for energy efficiency programs. After sealing, a duct blower test measures the remaining leakage. The goal is to reduce leakage to below 10% of total system airflow.